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The evidence for a general relationship between the speed of a diffusion pump and the molecular weight of the gas being pumped is reviewed. Experiments are described which show that when pumps are operated at their normal ratings, the speeds for hydrogen and helium may be considerably lower than for nitrogen, and it is established that this is due to the relative ease with which the lighter molecules diffuse from the fore-vacuum to the high vacuum side of the jet. The speeds of diffusion pumps for different gases are not in fixed ratios, but in ratios which depend upon the fore-vacuum pressure and the heat input to the pump boiler. At heater wattages exceeding twice normal the inverse (M)½ law may sometimes be obeyed.

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